The present invention relates to a scanning device such as an LSU (Laser Scanning Unit) mounted on a laser printer, and particularly to a scanning device capable of suppressing curvature of field caused by deformation of a lens.
In general, a scanning device includes a light source, a deflecting unit such as a polygonal mirror, and an image forming optical system such as an fθ lens. In the scanning device, a laser beam emitted by the light source is reflected and deflected by the deflecting unit and is converged onto a scan target surface such as an outer circumferential surface of a photoconductive drum so that a beam spot formed by the image forming optical system scans on the scan target surface in a main scanning direction. By on/off modulating the laser beam, a latent image can be formed on the scan target surface.
The scanning device to be used as an LSU is required to be low cost and compact in size. Therefore, in order to reduce cost and to downsize the scanning device, the scanning device is frequently configured such that the fθ lens is formed of two plastic lenses and the image forming optical system has a wide angle of view relative to its size in a direction of an optical axis. An example of this type of scanning device is disclosed in Japanese Patent Provisional Publication No. HEI 7-146437.
However, the above mentioned fθ lens formed of two plastic lenses has a drawback that if the fθ lens has a relatively wide angle of view, optical performance of the fθ lens tends to be deteriorated by a molding error because, due to the relatively wide angle of view, a first lens located on the polygonal mirror side in the fθ lens has a small radius of curvature and a relatively wide region of the fθ lens is used as an effective area. For example, if a meniscus lens is used in the fθ lens and the meniscus lens has a molding error, the meniscus lens tends to be deformed such that the curvature of each of lens surfaces thereof becomes larger. If the meniscus lens has such a molding error, the curvature of field also increases, and thereby the imaging quality may be deteriorated.